Currently available laser desorption techniques allow analysis of the chemical composition of surfaces at the micron level. However, conventional laser desorption techniques can be limited in their ability to desorb and ionize analytes present at the surface being analyzed. For example, preparation of dried sample spots on planar media is becoming a popular sample storage, preparation, shipping and analysis medium. In liquid extractive sampling of such prepared spots, fast, reproducible, and efficient reconstitution of the desired analyte is required to make direct analysis analytically viable. In such samples, the analyte of interest can sometimes crystallize, agglomerate, or take other intractable forms which are difficult to solubilize. Therefore, a need exists for improved surface extraction technology.
MALDI (matrix-assisted laser desorption/ionization) is a laser-based soft ionization method that has proven to be an effective ionization method for mass spectrometric analysis and investigation of large molecules. This method was developed in the late 1980s from such other desorption/ionization mass spectrometric methods as FAB (fast atom bombardment) and LDIMS (laser desorption ionization mass spectrometry). MALDI facilitates the production of intact gas-phase ions from large, nonvolatile, and thermally labile compounds such as proteins, oligonucleotides, synthetic polymers and large inorganic compounds by embedding these compounds in a chemical matrix. A laser beam (UV- or IR-pulsed laser) serves as the desorption and ionization source. The matrix absorbs the laser light energy and causes a small part of the target substrate to vaporize. The vaporized and ionized molecules are transferred electrostatically into a mass spectrometer where they are separated from the matrix ions and individually detected, usually by TOF (time-of-flight) mass spectrometry. The MALDI matrix must embed and isolate analytes, be soluble in solvents compatible with the analyte of interest, be vacuum stable, absorb the laser wavelength, cause co-desorption of the analyte upon laser irradiation, and promote analyte ionization.
A number of variations of the MALDI method are known. These include dried droplet, vacuum-drying crystallization, crushed-crystal, fast-evaporation, overlayer, sandwich, spin-coating, electrospray, quick & dirty (Q&D), matrix-precoated layers, chemical liquid, particle-doped (two-phase) liquid, chemical-doped liquid, solid supports, and MALDI on 2D-gels. In the case of samples that are insoluble it has been found that by pressing a mixture of finely ground sample and analyte, it is possible to record MALDI data from such compounds.
Electrospray is an alternative to MALDI. Electrospray generally involves flowing a sample liquid into an electrospray ion source comprising a small tube or capillary which is maintained at a high voltage, in absolute value terms, with respect to a nearby surface. The nearby (e.g. 1 cm) surface is commonly referred to as the counter electrode. Conventional ES systems for mass spectrometry apply high voltage (relative to a ground reference) to the emitter electrode while holding the counter electrode at a lower, near ground reference voltage. For the positive ion mode of operation, the voltage on the emitter is high positive, while for negative ion mode the emitter voltage is high negative. Liquid introduced into the tube or capillary is dispersed and emitted as fine electrically charged droplets (plume) by the applied electrical field.
The ionization mechanism generally involves the desorption at atmospheric pressure of ions from the fine electrically charged particles. The ions created by the electrospray process can then be used for a variety of applications, such as mass analyzed in a mass spectrometer.
In a typical ES-MS process, a solution containing analytes of interest is directed to the ES emitter which is held at high voltage, resulting in a charged solvent droplet spray or plume. The droplets drift towards the counter electrode under the influence of the electric field. As the droplets travel, gas-phase ions are liberated from the droplets. This process produces a quasi-continuous steady-state current with the charged droplets and ions constituting the current and completing the series circuit. A particularly useful application for electrospray is the production of gas phase ions from analytes in liquid solutions delivered by methods such as high pressure liquid chromatography, capillary electrophoresis or capillary electrochromatography to a system for detection and analysis, such as a mass spectrometer (MS).
Although ES MS has been known, the use of ES-MS for automatically reading out a plurality of spots, has been more recently developed. This is likely because of the technical challenges of sampling analytes from small spots on a sample surface with a liquid flow system in an automated way. Specifically, electrospray normally operates by having a sample dissolved in solution flow through transfer tubing to the ion source of the mass spectrometer. When trying to analyze a surface with electrospray, a significant challenge is presented in producing a probe suitable for transporting a normally solid-state surface sample into solution and then into the transfer line. In addition, a sophisticated structure is needed to control the alignment of the probe with the surface, the structure generally providing fine resolution of the probe movement relative to the surface.
Several methods for conducting surface sampling for electrospray mass spectrometry analysis, as well as other kinds of analysis, have been developed. Some such systems and methods are shown in US patents and Publications Nos. U.S. Pat. No. 6,803,566; U.S. Pat. No. 7,295,026, US 2010/0002905, and US 2010/0224013. The disclosure of these patents and publications is hereby incorporated fully by reference.
Once a way to sample the surface has been achieved, the next challenge is to dissolve the target sample analytes from the surface of interest. In liquid extractive sampling of such prepared spots, fast, reproducible, and efficient reconstitution of the desired analyte is required to make direct analysis analytically viable. In such samples, the analyte of interest can sometimes crystallize, agglomerate, or take other intractable forms which are difficult to solubilize. Therefore, a need exists for improved surface extraction technology.